1. Field of the Invention
The invention relates to medical implants and more particularly to implantable prostheses.
2. Prior Art
Medically implantable prostheses, exemplified by breast implants, are well known in the art. Such implants generally comprise a formed body presenting a nonreactive, biocompatible outer surface to surrounding tissue following implantation. The implant is recognized as a foreign body by the host's immune system and is encapsulated, walling the implant off from the rest of the host's body. As the capsule ages, molecular rearrangement within the capsule change the overall shape of the capsule. If the implant is deformable, such as is the case with fluid-filled prostheses, the shape of the implant will change to conform to the shape of the surrounding capsule. In many patients hosting fluid-filled prostheses, such as patients receiving a breast implant, the capsule slowly undergoes “spherical contracture”, palpably changing the shape and feel of the implant. Such spherical contracture is generally regarded as undesirable.
Fluid-filled medical implants generally comprise a viscous fluid contained within an elastomeric shell. It has been observed that fluid-filled medical implants presenting a smooth outer surface to surrounding tissue are particularly susceptible to deformation due to capsular contracture. In order to overcome, or at least minimize, the effect of capsular contracture, fluid-filled medical implants have been developed that have a textured outer surface. The irregular topography of the outer surface apparently induces molecular disorganization in the capsule that forms around the implant following implantation. Such molecular disorganization in the capsule is believed to resist spherical contracture.
In order to texture the outer surface of implants, two methods are commonly used in the art. In a first method, described by Quaid in U.S. Pat. No. 4,889,744, a layer comprising a biocompatible, nonbioabsorbable uncured silicone elastomer is applied to the outer surface of a silicone implant. Before the layer is permitted to cure, solute particles, usually salt, are embedded in the (tacky) outer layer. The outer layer bearing the solute particles is then partially cured and exposed to an appropriate solvent to remove the solute particles. The outer layer is then fully cured. The plurality of voids remaining in the layer following removal of solute and curing have an open celled structure. The resulting medical implant has both a textured outer surface and unitary construction. An alternative embodiment of the first method comprises the formation of outwardly projecting fibrils on the outer surface. Again, the fibrils comprise a nonbioabsorbable material.
In a second method, as disclosed by Brauman in U.S. Pat. No. 4,648,880 and RE35,391, an implantable prosthesis comprises a flexible container with a soft gel or fluid filling and an outer plastic or polymeric covering bonded to, and substantially enveloping the flexible container. The outer plastic polymeric covering is made from a woven mesh fabric such as Dacron® (poly(ethylene glycol terephthalate)) or Teflon®, and has numerous pores or interstices within the material as well as a rough textured external surface. The outer plastic covering, comprising a nonbioabsorbable mesh, such as woven Dacron® fabric, is glued to the outer surface of the implant to provide a rough textured outer surface. While this type of implant provides a textured outer surface that is functionally similar to implants made by the first method, the implant lacks unitary construction which can lead to structural failure, such as delamination, following implantation within the body.
Either of the foregoing prior art texturing methods have advantages and disadvantages. While both methods appear to provide an implant that resists capsular contracture, the second method provides an outer layer that may delaminate within the body. For this reason, the first method of making an implant having a textured outer surface is currently favored over the second method. In the first method, the outer tissue contacting surface of the implant provides means for disrupting and/or disorganizing the orderly alignment of the structural proteinaceous biopolymers comprising the capsule during capsule formation. The outer surface, when textured in accordance with the first method, also permits capsular tissue to grow into the plurality of (open celled) interstices in the outer surface of the implant. The partial or total adhesion of the implant to the capsule due to such tissue ingrowth may be undesirable in the event it becomes necessary to remove or replace the implant. Further, partial or asymmetric adhesion between the capsule and the outer surface of the implant may give rise to undesirable cosmetic effects. Notwithstanding the foregoing disadvantages, textured implants having a biocompatible, nonbioabsorbable outer tissue-contacting surface are generally considered to reduce the incidence of capsular contracture in patients. There remains a need for an implantable fluid-filled prosthesis that resists capsular contracture following implantation and that resists adherence of the implant to the capsule.